Magnetic structures for high frequency energy interchange apparatus



Dec. 27, 1960 w. R. TURNER 7 2,966,609

MAGNETIC STRUCTURES FOR HIGH FREQUENCY ENERGY INTERCHANGE APPARATUS Filed Nov. 22, 1957 2 Sheets-Sheet 1 m ENTOR.

Dec. 27, 1960 A w R URNER 2,966,609

MAGNETIC STRUdTUlES FOR HIGH FREQUENCY ENERGY INTERCHANGE APPARATUS Filed Nov. 22, 1957 2 Sheets-Sheet 2 l V/z so/vffi? EENEE r]: E 5 INVENTOR.

BYMW

2,966,609 1 Patented Dec. 27, 1960 MAGNETIC STRUCTURES. FOR HIGH FRE- QEENEY ENERGY INTERCHANGE APPA- n TU Wilson Reese Turner, Mountain View, Calif., assignor to General Electric Company, a corporation of New York Filed Nov. 22, 1957, Ser. No. 698,126

6 Claims. (Cl. 31'535) This invention is directedtto apparatus including magneticstructures and more particularly to such apparatus including magnetic structuresfor high frequency energy interchange devices wherein'energy interchange is dependent upon interaction between an electron stream and radio frequency fields.

The electron tube known as a traveling wave tube is an example of such a device; In a conventional traveling wave tube, an elongated section of transmission line is mounted within an evacuated envelope; An input radio frequency wave is coupled onto the transmission line which is designed as a delay line or slow wave struc ture along which electromagnetic waves are propagated at some relatively small fraction of the velocity of light. In the most conventional form, the delay line is a con ductive helix such as a helical metal coil. An electron stream is projected along apath near the helix in such a manner that the electronsin the stream and the radio frequency fields produced by the helix travel in close proximity. The velocity of the electron stream is made substantially equal to or synchronous with the axial component of the wave velocity along the helix. In operation, radio frequency waves traveling. along the trans mission line interact with the electronsin the stream to cause a redistribution of electrons in the form of partial bunchingalong the stream.- As the wave and beam netic fieldsalong the interaction region wherein the magnetic fiux (lines of force) are perfectly straight is common to the: energy interchange devices of the type under consideration here and is the problem to which the present invention is -directed.

It is well known that parallel magnetic pole pieces having cross sectional dimensions which are of the order of, or smaller than, the distance between them produce a magnetic field between them which varies rapidly with position along any straight line between them.

This is true even along a straight line between the centers of the pole faces. This situation arises because of the fact that the magnetic lines of force bow out around the pole pieces. This effect is known as fringing. Due to the defocusingyaction of fringing flux lines, some means must'be provided to straighten the flux lines in the interactionarea if the magnetic field between such pole pieces is to be used for focusing an electron stream.

One solution to the problem is to use magnets with pole pieces which are considerably larger in cross sectional dimensions than the pole separation in order to obtain the required field configuration necessary to focus an electron stream. This'approach to the problem is objectionable since the diameter of the electron stream to be focused is typically between 1% and 10% of the length of the electron stream (the length of the interaction region). Thus, using this solution to the problem, an extremely large volume of magnetic field is required to focus a-stream with intrinsically small volume.

Accordingly, it is an object of this invention to provide a means of straightening flux lines or reducing fringing of the magnetic field between a pair of relatively widely spaced pole pieces of comparatively small transverse dimensions thereby to reduce the spreading tendency of the electrons in an electron stream directed axially along the magnetic field.

travel synchronously along the helix, the inverse phenomenonoccurs and the bunched beaminduces fields and. currents along the helix. The amplitude of the wave increases along the helix because the electron stream gives upmore energy to-the helix than-its abstracts fr0m-it. Consequently, an amplification of the radiofrequency wave on the helix takes place; The region in which the electron stream and radio frequency fields. interact'iscalled the interaction region.

If the. energy interchange described aboveisto take place, the'electron -stream must pass'through the-inter-" action region in: close proximity to-the radio frequency field produced by .the transmission line. For most effective operation, electronsfrom the stream should be ccnimedto the interaction region throughout its length. Sincethe space charge of the electrons in the stream tends to force them apart, some.-means must be provided toafocus the stream throughout the interaction area to prevent-the electrons' from speading and passing out of the-interaction area Theusual method of focusing is to provide a unidirectional magnetic. fieldhaving lines of force in the direction of the electron stream, byemploying a solenoidcwhichsurrounds the entire traveling wave tube structure and produces an axial magnetic field- Another means of focusing the electron stream is to generate a periodic magnetic fieldalong the axis of thetraveling wavetube either by means of solenoids or by. specially designed permanent magnets. In anycase, if. themagnetic. lines of force. produced. along-the axis of the tubeare not straight ('e.g'. ifathey bow radially The use of a large solenoid for providing the magnetic field along the axis of a traveling wave tubeis generally satisfactory. However, the solenoids required are extremely large and heavy and require considerable power in order to provide the necessary axial magnetic field. The periodic focusing schemes either by solenoids'or by specially designed permanent magnets may also be satisfactory'for certain applications; however, they may depend-upon precisetube configuration, and may betoo I complicated, cumbersome, and unwieldy for general outwardly), they may bedetrimentaltothe focusing-of the electron stream. The problem of producing magusage.

Accordingly; it is a'furthe'r object of this invention to provide ameans for decreasing variations in the mag? netic focu'singfield along. the axis of a high frequency energy interchangedevice when relatively widely spaced pole pieces of small transverse dimensions are used to produce the focusing-- magnetic field.

Incarrying out the present invention; the magnetic lines of force. between relatively widely spaced pole pieces of smalltransverse dimensions are straightened by placing discontinuous magneticallypermeablemembers between pole pieces of themagnet. That is to say that the magnetic fieldbetween such pole pieces is'made nearly constant by utilizing specially shaped magnetically permeable members between pole faces.

The novelfeatures-which are believed to becharacteristic of this invention are set forth with particularity in the-appended claims. The'invention itself both as to its organization and methodof operation together with further objects and advantages thereof may best be understood by: reference: to the following description taken in connectionwith the accompanying drawings in which:

Figure 1- is a perspective 'viewof a magnet and magnetic; field shapingmeansrin: accordance with an embodiment of-this invention;

Figure 2 is a side elevation of a pair of magnetic poles showing schematically the fringing magnetic flux lines between the poles in the absence of intervening magnetic materials;

Figure 3 is a side elevation similar to that of Figure 2 but of a slightly different scale, showing flux shaping members utilized in accordance with the teachings of the present invention and illustrating their action;

Figure 4 is another side elevation similar to those of Figures 2 and 3 having the center portion broken away and illustrating more widely separated magnetic poles which require additional flux shaping members; and

Figure 5 is a central vertical longitudinal section through a traveling wave tube utilizing a magnet similar to the one illustrated in Figure 1 and the means of making the magnetic field strength constant along the axis of the traveling wave tube illustrated in the embodiment of Figure 1.

As illustrated in Figure 1, the field producing permanent magnet has the configuration of a hollow ball flattened on opposite ends and having a pair of pie shape sections 8 and 9 removed from opposite sides of the magnet to allow the introduction of wave guide (not shown). Another way to describe the configuration of the permanent magnets illustrated is in terms of a surface of revolution as defined when a C lying on its side in such a manner that its open arms lie in a vertical position is rotated about a horizontal axis which extends through the outer extremities of the open arms of the 6C?! The particular magnet 10 illustrated is one which was used in the application described herein and therefore is the one selected to illustrate the present invention. However, the invention is not limited to the use of such a structure. The pole pieces 11 and 12 for the magnet 10 are thick portions of magnetic material at the opposite flattened ends and on the axis of revolution of the C which defines the general shape of the magnet. It will be seen that one pole piece 12, the pole piece to the right in the illustration which is considered a south magnetic pole (see Diagram, Figure 2), has a centrally located aperture 13 therethrough for the insertion of the envelope of a traveling wave tube as is more fully described in connection with Figure 2.

As previously indicated, parallel magnetic pole pieces, such as pole pieces 11 and 12 of magnet 10, which have cross sectional dimensions of the order of, or smaller than the distance between them produce a magnetic field between them which bows outwardly. This is illustrated by magnetic lines of force or flux 14 and 14a in Figure 2 of the drawings. Flux lines at the outer edges of the poles 11 and 12 such as lines 14a are especially bowed. In order to straighten the lines of force 14 and thereby eliminate the defocusing action of the fringing magnetic field, and also to increase the useful magnetic flux between the pole faces 11 and 12, a pair of magnetically permeable flux shaping members 15 and 16 are provided (see Figure 3). Each of the magnetic flux shaping members 15 and 16 have the general shape of a frustum of a right circular cone and each is hollow with its inner surface substantially concentric with its outer surface. Further, both the inner and outer surfaces of the fluxshaping members 15 and 16 are continuous in the sense that they do not contain sharp bends or angles. In view of the conical shape of flux straightening members 15 and 16, they are referred to as flux cones. The flux cones l5 and 16 are best seen in Figures 1, 3, 4, and 5.

In the arrangement illustrated in Figure 1, the flux cones 15 and 16 are held in position between the pole pieces 11 and 12 by means of a flux cone supporting cylinder 17 of non-magnetic material, such as brass, which is fitted between the pole pieces 11 and 12 of the magnet 10 in such a manner that its longitudinal axis coincides with a line between the centers of the pole faces 11 and 12 (the axis of the magnetic structure). The flux cone supporting cylinder 17 has its opposite ends inserted in recesses 19 in the inside face of the pole pieces in order to hold it in position. A slightly enlarged portion is provided along the center portion of the flux cone supporting cylinder 17 so that the flux cones 15 and 16 may he slipped over the ends of the cylinder 17 and force fitted in position thereon. Thus, the cones are held in position with their axes coincident with the axis of the cylinder 17 and their apices directed toward the center of the structure (toward each other). In other words, the base of each of the flux cones 15 and 16 is nearest the adjacent pole face and the apex of each flux cone is directed away from the nearest pole face.

As illustrated, the angle (see angle 0 of Figure 5) of the slant height (sides) of the flux cones 15 and 16 relative to their axis is approximately 45 It has been found that angles between the cone sides and axes of between 30 and 60 mechanical degrees are satisfactory. However, for the particular application illustrated, it was found that the 45 angle was an optimum for providing a constant flux density or uniform magnetic field in the area of interest between pole pieces 11 and 12. In one model it was found that without the use of flux cones, the flux density between pole pieces 11 and 12 varied :25% Whereas the addition of the flux cones reduced the variation to i3%.

The flux straightening action of the cones 15 and 16 may best be described in connection with the somewhat schematic views of Figures 2 and 3. Figure 3 is a cross sectional view of the elements of the magnetic circuit illustrated in Figure 1 with the associated magnetic lines of force 14. Figure 3 differs from Figure 2 only in the scale chosen and the addition of the flux cones 15 and 16 which change the contour of the magnetic lines of force 14. As illustrated the lines of force 14a near the outer edge of the pole pieces 11 and 12 which tend to bow out are intercepted by the flux cones 15 and 16 and guided into the center of the structure (the interaction region). Thus, the magnetically permeable flux cones 15 and 16 perform the dual function of concentrating and decreasing variations in the magnetic field between the pole pieces 11 and 12. The concentrating function increases the useful flux between the pole pieces and the variation decreasing function eliminates the electron stream defocusing action caused by fringing lines of fiux.

The flux cones are effective to shape the flux lines 14 in this manner due to the fact that they are constructed of a material of high magnetic permeability, such as cold rolled steel. Since the high permeability material provides a path of low resistance for magnetic flux, such a material tends to collect magnetic flux lines in the area. Further, magnetic force lines both enter and leave a magnetic material at right angles to its surface. Thus, as illustrated in Figure 3, the fringing magnetic lines of force 14a between the pole pieces 11 and 12 are collected by the flux cones 15 and 16 and enter the inside surface at right angles thereto, are guided along the cones toward their apices and leave the cones close to the axis of the structure. The walls of the flux cones 15 and 16 are preferably thick enough that they do not saturate appreciably and, as illustrated, they are of uniform thickness so that the cones do not saturate unevenly. However, for particular magnetic circuit designs, it may be'desirable to vary the wall thickness of the cones and such arrangements do not depart from the spirit and scope of the present invention.

For magnetic circuits where relatively small magnetic pole pieces are spread even farther apart than those illustrated in Figures 2 and 3, it may be desirable to use more than one pair of flux cones. This is true for two reasons. The first reason is that flux lines may again start to bow out between cones and it may be necessary to provide some means to prevent this fringing. The second reason for utilizing additional flux cones is to collect and direct additional flux lines into the interaction region between the pole pieces 11 and 12.

Figure "4 illustrates an arrangement wheretwov additional flux cones 20 and 21 are used in thema'gnetic cir cuit. The magnetic pole pieces andthe pair of'fiux cones nearest these pole pieces-are given the same reference numerals as thecorresponding'elements of Figure '3since they are similar in configuration, placement and function. As illustrated in Figure 4, the two additional "flux cones '20 and '2 1 also have the configuration of frusta' of right circular cones. The are positioned between thefirstpail of'fiux cones 15 and 16 with their'axes concentric and with their apices directed toward each other. Theadditional flux cones 2t) and 21 have base circles when are consider-ably larger than the base circles of the first pair of'flux cones 15 and 16 in order to collect fringing-marg netic lines of force 14b not collectedby thefirst pair of flux cones 1'5 and 16 and guide them in toward the central longitudinal axes of the structure. Froman inspection of Figure 4, it may'also be seen how the additional magnetically permeable flux cones 20. and 21 also collect and guide the fringing magnetic lines of force 1411 back toward the longitudinal axis of the magnetic circuit.

Thus the flux cones 15, 16, ZO-and 21 all decrease variations inthe magnetic field between the pole faces 11 and 12. It is obvious that-more such conestmay be used where desired ornecessary, the relative sizes'of the cones may be varied, and the cones'm'ay'be designed such that the wall thicknesses vary so that saturation is produced in some partof the'fiux cone. All of'lthese variations may be made without departing from-the spirit of the present invention. These variations are not all illustrated since the design variations are dependent upon theparticular use. To obtain precisedata for an-individual application, resort-should-be hadto a'well'known technique" of using a-fluxplotting analogsuchas maybe set'upin an electrolytic tank. For suchtechniques see the book, Electromagnetic FieldsTheory and Applications by Weber, volume I-Mapping of Fields, pages 187- 193, published .by John Wileyand Sons, Inc., New York, New York, 1950 and the article entitled Electrolytic Tank Design and Applications by Phyllis A. Kennedy and Gordon K'entzin Review oLScientificInstruments, volume 27, No. 11, pages 916-927, November 1956.

In Figure 5, the magnetic circuit is utilized in conjunction with a conventional traveling wave tube 22. In this figure, only a portion of the magnet 10 is illustrated. As previously described, the flux cones and 16 are held in position between the magnetic pole faces 11 and 12 by means of the non-magnetic cylindrical spacer 17.

The traveling wave tube 22 is provided with an envelope 23 that is generally long and cylindrical with an enlarged cylindrical portion 25 at one end which houses an electron stream producing gun 26 and a small slender cylindrical portion 27 which houses a long main interaction transmission line 28 which as illustrated is a helix. The small slender cylindrical portion 27 is positioned with its longitudinal axis in alignment with the longitudinal axis of the magnetic structure so that the magnetic field produced along this axis may be used to focus an electron stream directed along the longitudinal axis of the tube 227 In order to allow the tube 22 to be so positioned, apertures 13 and 24 (see Figure 5) are provided in pole pieces 11 and 12 respectively, the elongated portion 27 of tube 22 is then inserted through the apertures 13 and 24 in pole pieces 11 and 12 and the enlarged portion 25 of the tube 22 is positioned within a tube receiving recess or aperture 31 in the outer face of the magnetic pole piece 11 on the left hand side of the structure.

The electron gun 26 housed in the enlarged portion 25 at one end (the left as illustrated) of the evacuated envelope 23 forms and directs a stream of electrons along.

the axis of the envelope. The electrons which are formed into a stream are emitted from a cathode 33 in response to heat applied thereto by a heater 34. The heater 34 is illustrated as a high resistance coil member which must be connected to a source of voltage (not shown) in order toproduce heat necessary to make the "cathode 33 emit. The electronstream 32 is formed and projected along the axis of the tube by a pairof centrally apertured electron stream focusing and accelerating electrodes 35 and 3'6'wliich are located in the enlarged portion 2 5 of the tube 22 between the electron emitting cathode member 33 and theb'eam helix or main transmission line 28. Since the beam helix 28 is'positioned inside the slender portion '27 of the envelope 23 with its axis coincident with the tubeaxis, the electron stream flows down the length of the'tube'in close proximity thereto, thus providing an interaction region along the tube axis. A collector anode 37 is positioned at the opposite or output end of the structure to dissipate residual 'energy in thestream.

Electromagnetic waves are introduced'onto the beam helix 28 by means ofa conventional wave guide 40'Whi0h is brought in through one of the removed pieshaped sections8'and' 9 (best seen'in' Figure 1) of magnet structure 10 and through an aperture 4 1 in the end of flux cone supporting cylinder 17 near the electron gun end of the tube 22. In order to transfer the electromagnetic wave to the beam helix, the elongated portion 27 of tube 22 is'positioned within an aperture 39 through input waveguide 40. The amplified electromagnetic wave is extracted from the beam helixZS at the opposite end of the tube by another waveguide 42 which is also brought in through the magnet structure 10 as described with respect to the input waveguide 40'. Another aperture 43 is provided in the opposite end of the flux cone supporting cylinder 17 to allow the output waveguide 42 to be introduced. The slender elongated portion 27 of tube 22 passes through an aperture 44 in the output waveguide 42.

The electromagnetic wave introduced by means of input waveguide 40 is propagated down the beam helix 28 in close proximity to the electron stream 3-2 produced by the electron gun 26. The electromagnetic wave so prop agated is amplified through. the mechanism of interaction with thezelectron stream 32 and the amplified electromagnetic Wave is extracted through output wave guide 4-2.

The permanent magnetic focusing member 10 produces afmagnetic field axially along the tube 22 which is sufficiently strong to prevent the space charge of the electrons in the stream from spreading them to such an extent that they pass out of the interaction region and/ or intercept the helix. Since the magnetically permeable flux cones 15 and 16 decrease variations of the magnetic field between the pole pieces 11 and 12 and since the axis of the tube 22 coincides with a line between centers of the pole pieces 11 and 12, the flux cones 15 and 16 serve to make the magnetic field constant along the axis of the traveling wave tube 2 2. In this manner, the problems attendant on providing specially shaped magnets or on designing periodic focusing arrangements into the traveling wave tube is circumvented since the magnetic path external to the poles 11 and 12 may take any convenient shape. Also the magnet Weight may be reduced as compared to the case in which pole pieces of larger cross sectional dimensions in the gap are used since a smaller volume must be driven by the field producing magnet 10.

While a particular embodiment of the invention has been shown and described, it will of course be understood that the invention is not limited thereto since many modifications both in the magnetic circuit arrangement and in the instrumentalities employed may be made. It is contemplated that the appended claims will cover any such modifications as fall within the true spirit and scope of this invention.

What I claim is new and desire to secure by Letters Patent of the United States is:

1. A high frequency energy interchange device including an elongated evacuated envelope, electron gun means at one end of said envelope for projecting a stream of electrons along the length of said envelope, collector means at the opposite end of said envelope to dissipate residual energy in said electron stream, an elongated section of transmission line mounted within the envelope between said electron gun means and said collector means in coupled relationship with the electron stream, and magnetic circuit means positioned coaxially with respect to said evacuated envelope to produce a magnetic field along the length of said envelope to provide focusing action for said electron stream, said magnetic circuit means comprising a pair of magnetic pole pieces and at least a pair of flux straightening members of a material having a high magnetic permeability, said flux-straightening members having the configuration of hollow cone frusta with continuous inner and outer surfaces without angles therein and being positioned coaxially between the pole pieces with the apices of the cones directed toward each other.

2. A high frequency energy interchange device including an elongated evacuated envelope, electron gun means at one end of said envelope for projecting a stream of electrons along the length of said envelope, collector means at the opposite end of said envelope to dissipate residual energy in said electron stream, an elongated section of transmission line mounted within the envelope between said electron gun means and said collector means in coupled relationship with the electron stream and magnetic circuit means positioned coaxially with respect to said evacuated envelope to produce a magnetic field along the length of said envelope to provide focusing action for said electron stream, said magnetic circuit means comprising a pair of relatively widely spaced magnetic pole pieces having cross sectional dimensions which are of the order of, or smaller than the distance between them including a plurality of pairs of coaxial flux straightening hollow cone frusta of a material having a high magnetic permeability between the pole pieces, each one of each pair of hollow cone frusta having continuous inner and outer surfaces without angles therein and being equidistant from an opposite pole piece with the apex of the cone directed toward the other one of said pair, and each pair of fiux cones being positioned a different distance from said pole pieces.

3. Apparatus as set orth in claim 2 wherein the pairs of flux straightening cone frusta which are displaced by progressively greater distances from the pole pieces have progressively larger bases.

'4. Apparatus for decreasing variations of the magnetic field between a pair of magnetic pole pieces including at least a pair of fiux straightening members of a material having a high magnetic permeability and having the configuration of hollow cone frusta, said pair of magnetically permeable members having continuous inner and outer surfaces without angles therein positioned coaxially between the pole pieces with the apices of the defining cones directed toward each other.

5. Apparatus for decreasing variations of the magnetic field between a pair of relatively widely spaced magnetic pole piecw having cross sectional dimensions which are of the order of, or smaller than the distance between them including a plurality of pairs of hollow coaxial flux straightening cone frusta of a high magnetic permeability material between the pole pieces, said hollow cone frusta having continuous inner and outer surfaces without angles therein, each one of each pair of cone frusta being equi-- distant from an opposite pole piece with the apex of the cone directed toward the other one of said pair, and each pair of flux cones being positioned a different distance from said pole pieces.

6. Flux straightening apparatus as defined in claim 5 wherein the pairs of flux straightening cone frusta which are displaced by progressively greater distances from the pole pieces have progressively larger bases.

References Cited in the file of this patent UNITED STATES PATENTS 2,259,531 Miller et al. Oct. 21, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,791,718 Glass May 7, 1957 2,807,743 Ciotfi Sept. 24, 1957 2,811,663 Brewer et al Oct. 29, 1957 2,812,470 Cook et al Nov. 5, 1957 2,841,739 Pierce July 1, 1958 2,844,750 Veith et al July 22, 1958 FOREIGN PATENTS 153,259 Australia Sept. 17, 1953 UNITED STATES PATENT QFFICE nfcember 27 1960 Wilson Reese Turner It is hereby certified that err ent requiring correction and that th e numbered patcorrected below.

should read as Column 7,, line 13, strike out "and losing o Signed and sealed this 23rd day of May 196io (SEAL) Attest:

ERNEST W. SWIDER DAVID L, LADD Attesting Officer Commissioner of Patents 

